The ability to manufacture fluid products such as cream, milk, etc. from concentrated and dried ingredients significantly enhances processing flexibility. Such a process facilitates the operation of manufacturing plants at optimum capacity, particularly in areas unable to provide a sufficient or continuous milk supply. Producing fluid products from dry or concentrated ingredients also minimizes the shipping costs for transporting such ingredients, while eliminating the need to store such ingredients at refrigeration temperatures. Further, such a process permits the utilization of stockpiled surplus dry products such as nonfat dry milk, enhances the efficiency of producing products such as plastic cream, and creates effective components for standardizing milk.
Realizing the desired process requires the development of a successful procedure for rehydrating the protein supplied by the dried ingredients, simultaneously combined with a method for creating the desired emulsion. The manufacture of concentrated and dried ingredients both dehydrates the proteins and destroys the existing fat emulsion. Merely adding water to rehydrate the protein using conventional systems frequently is not sufficient to establish the desired fat emulsion needed to produce cream, milk, or related fluid products.
Emulsions are systems created by the dispersion of a liquid as droplets within another, otherwise immiscible liquid. All fats and oils, including milk fat, are inherently immiscible in water, so food emulsions are generally defined as “oil-in-water” or “water-in-oil” systems. The emulsion naturally occurring in milk and cream is a dispersion of milkfat droplets, 0.1 to 10 μm diameter, within the milk serum (Walstra and Jenness Dairy Chemistry and Physics John Wiley & Sons, New York p. 5, incorporated herein by reference). The milk serum is the portion of milk containing the water and the water-soluble components. The production of highly concentrated or purified milk fat products such as butter, clarified butter, ghee, anhydrous milk fat, various purified milkfat blends, etc., inherently destroys the natural milk fat emulsion.
Various types of high-pressure, high-shear mixers, such as homogenizers, create emulsions with physical force as a mixing agent. The effectiveness of these mixers is directly proportional to the amount of energy applied to the system to provide the required force. Hence, the quantity of fat that can be emulsified in a given amount of water and the size of the resulting droplets in the finished dispersion depends upon the amount of energy supplied by the mixer. Many mixers cannot supply enough energy or force to successfully emulsify the fat, particularly in high fat, low protein products, such as plastic cream.
Although high-pressure, high-shear mixers may initially create an emulsion, the ability to maintain that emulsion typically depends upon the action of emulsifiers. Emulsifiers are compounds that simultaneously interact with the two immiscible liquids, the water and oil, to reduce the interfacial tension. Compounds that are effective emulsifiers maintain two separate, independent molecular sections. One section of such molecules must be miscible in water, and is frequently defined as being “hydrophilic,” while the other section must be miscible in oil or fat, and is termed “hydrophobic.” This simultaneous interaction of the hydrophilic section in the water and the hydrophobic section in the oil or fat, positions the emulsifier at the interface between the oil and water, preventing the water and fat from separating back into independent phases.
Although several compounds are effective emulsifiers, legal regulations frequently prohibit the use of many of these compounds in the manufacture of most dairy and numerous food products. Many highly effective emulsifiers are not allowed in the manufacture of cheese varieties with a standard of identity, such as Cheddar, Mozzarella, Colby, Swiss etc. The only viable alternative is to create the required emulsion with allowed ingredients and/or with indigenous components. Proteins generally are the indigenous component most likely to effectively emulsify fat in the production of cream, milk, and related products.
Proteins that are effective emulsifiers rapidly diffuse to the interfacial area between the two immiscible liquids and partially denature, uncoiling to expose separate sections of predominantly hydrophilic and hydrophobic amino acids. The section of the protein primarily consisting of hydrophilic amino acids interacts with the water, while the section of the protein primarily consisting of hydrophobic amino acids interacts with the fat to create and maintain the emulsion. The effectiveness of specific proteins at emulsifying fat depends upon the possession of separate hydrophilic and hydrophobic sections of amino acids in the primary structure, the availability of a sufficient amount of protein, the solubility of the protein, and the protein's conformation, or shape.
Protein conformation is particularly critical to emulsification, as the protein must initially possess the separate hydrophilic and hydrophobic sections that are large enough to effect emulsification. The protein conformation subsequently must expose the proper amino acid section to the respective liquid, water or oil. Native proteins inherently position the hydrophilic sections to interact with water. However, the native conformation of most proteins minimizes the exposure of the hydrophobic amino acid sections to water. Such proteins may not be effective emulsifiers unless modified to promote the interaction of the hydrophobic sections with the oil.
Protein conformation is determined by the nature of chemical bonds and interactions. These bonds include covalent disulfide bonds, ionic bonds with various ions and salts, and hydrophobic/hydrophilic interactions with the solvent. The creation and maintenance of these bonds depends upon environmental parameters of the solution including the pH, temperature, pressure, overall ionic strength, and the identity of the ionic species. Controlled adjustments in these environmental parameters may therefore transform specified proteins into significantly more effective emulsifiers.
Modifications in protein conformation that enhance the ability of the protein to emulsify fat do not eliminate the need for high-shear mixing. Indeed, high-shear mixing, or homogenization, is still required to disperse the two immiscible components (fat and water). However, conformational modifications that transform proteins into superior emulsifying agents will proportionally enhance the ability of the high-shear mixing to produce emulsions, reduce the amount of shear needed to create an emulsion, increase the ability of the system to maintain the desired emulsion, and minimize the amount of protein needed to emulsify a specified quantity of fat or oil. The desired changes in protein conformation may facilitate the formation of emulsions that can not be produced and maintained by shear forces alone, such as the production of plastic cream.
Milk proteins constitute a primary protein source for emulsifying fat in many food products. Solubility differences at pH 4.6 divide the milk proteins into two major groups, the caseins and the whey proteins. The whey proteins are a mixture of globular proteins that remain soluble at pH 4.6 and account for about 20% of the total milk protein. The caseins are a group of phosphoproteins that precipitate from raw skim milk at pH 4.6, and comprise the remaining 80% of the milk proteins.
Caseins in general, and β-casein in particular, possess a highly discrete distribution of hydrophilic and hydrophobic amino acids that favors emulsification. However, most of the casein in milk is bound together as colloidal particles, called micelles, with significant amounts of insoluble calcium or magnesium phosphate salts. The aggregation of caseins into the micelle structure significantly limits the ability of the individual caseins to emulsify fat, effectively negating the favorable amino acid distribution in the primary structure of these proteins. Replacing the divalent cations calcium and magnesium in casein micelles with an appropriate monovalent cation, such as sodium, releases individual caseins as sodium salts. Such caseins are excellent emulsifiers. That is, the modification of casein conformation that occurs when the micelles are transformed to produce independent casein salts with monovalent ions significantly increases the ability of these proteins to emulsify fat. Berger et al. illustrate this principle in describing the use of emulsifying salts to enhance the emulsifying ability of casein in process cheese manufacture (Berger et al. Processed Cheese Manufacture: A JOHA Guide BK Giulinin Chemie GmbH Co. OHG, Ladenburg pp. 51-61).